This invention relates generally to the field of remote controlled gain devices, and more particularly voltage controlled amplifiers employing operational transconductance amplifiers.
Broadly speaking a remote controlled gain device is a device useful for effecting a change in the amount of amplification of a signal, termed the input signal, from a point of control removed from the location at which the actual change in amplification is occurring.
The ability to remotely control gain is desirable for a number of reasons. In the control of voltages associated with audio frequencies, for example, it is frequently desirable to control the amount of amplification of an input signal from a remote location. Without the use of a gain device which could be remotely controlled, this would require the routing of the input signal to the remote location for direct adjustment thereon. Such remote routing of an input signal is highly undesirable as it permits the possibility of compromising the integrity of the input signal. In particular, the remote routing of an input signal permits exposure of the input signal to unwanted signals which would thereafter be included with the input signal. Once this has occurred it is not possible from a practical standpoint to remove the unwanted signal or its effects on the input signal, without an effect therefrom remaining on the input signal.
In addition to the possibility of exposure of the input signal to unwanted signals, the remoting of an input signal usually results in a degrading of the input signal from the remoting process itself. This frequently occurs due to the effect of the capacitance of the wire used to physically transport the input signal to the remote location. The ultimate effect of such a capacitance effect is a loss of high frequency information.
Consequently, when it becomes necessary to remotely control the gain of an input signal, it is desirable to avoid remoting the input signal itself, and instead remote a control signal which will indirectly control the gain of the input signal.
Remote controlled gain devices find use in a wide variety of applications. Professional audio mixing boards frequently have the point of control removed from the associated electronics. Consequently voltage controlled amplifiers are used to control the actual desired gain applied to the particular input signals by the use of a control voltage which is adjusted from a remotely located control panel.
Professional audio and video tape recorders also frequently employ the use of a voltage controlled amplifier in the control of various signals, as it is often desirable to strictly limit the routing of an input signal. Consequently, voltage controlled amplifiers are frequently used, being placed directly on the printed circuit board, and routing only the associated control signal to the recorder control panel.
Voltage controlled amplifiers also find use in signal processing applications wherein it is necessary to control the gain applied to an input signal in response to a complex electrical signal which has been derived as the result of other signal processing. In particular, audio signal processing equipment such as expanders and compressors use voltage controlled amplifiers. In such equipment, the amount of gain applied to an input signal is dynamically varied in response to the results of a complex mathematical algorithm being performed on the input signal.
In the past, voltage controlled amplifiers have been implemented using the basic principle of a resistive voltage divider network. This approach is based on the fact that the drain to source characteristics of a field effect transistor are essentially resistive in nature, in an amount determined by the potential applied to the gate. Consequently a voltage controlled attenuator can be implemented by employing a single resistor in series with a field effect transistor. In particular, the signal of interest would be applied to one terminal of the resistor, with the second terminal connected to the drain of the field effect transistor. The source of the field effect transistor would be connected to ground, with the control voltage applied to the gate. The attenuated output signal would then appear on the drain terminal of the field effect transistor.
Consequently by varying the control voltage applied to the gate of the field effect transistor, the effective d.c. resistance which is shunting the input signal to ground is correspondingly controlled. The result is a variation in the amplitude of the input signal in accordance with variations in the amplitude of the control signal.
Such an implementation of a voltage controlled amplifier has a number of characteristics of interest.
First, as there is an absence of active devices in the signal path, a very large dynamic range is possible for the input signal. Dynamic ranges in excess of 100 db are common.
While a large dynamic range is possible for differences in amplitude of the input signal, the corresponding amount of control is significantly less. In particular the range of control over an input signal is typically approximately 40 db. This results from the parameters which are associated with the devices. In particular, typical values for the first resistor are 10 K ohms, and typical drain to source ON resistance for field effecting transistor is 100 ohms. As the amount of gain control possible in this approach is dependent upon the ratio of these two resistances, clearly the limiting value is drain to source resistance for the field effect transistor.
As the principle involved is basically one of a resistive voltage divider network, and the limiting factor the drain to source ON resistance of the field effect transistor, an improvement in the amount of gain control is possible by resistively coupling a second field effect transistor in parallel with the first, and taking the attenuated output signal from the drain terminal of the second field effect transistor. The control signal is applied simultaneouly to the gates of the first and second field effect transistor. Such an arrangement results in a decrease in the effective resistance which shunts the input signal to ground. In such an arrangement it is possible to achieve a control range of 80 db for the control signal.
Further characteristics associated with the above described implementation of a voltage controlled amplifier include an exponential relationship between the control signal and the effect on the input signal. This is a desirable result, as the response of the human ear to differences in loudness is exponential.
A further characteristic of such a voltage controlled amplifier is the nature of second harmonic distortion. The amount of second harmonic distortion present in the output signal is not constant, and tends to increase with increase in amplitude of the input signal. This is an undesirable result.
An additional characteristic is of interest. As the above implementation is basically one of a resistive voltage divider, such a device cannot provide gain. To the contrary, only attentuation of the input signal is possible, in amounts determined by the magnitude of the control signal.
A final characteristic is of practical importance. As a semiconductor device is used, the resulting device exhibits a temperature dependence. In particular, the amount of attenuation on the input signal is in part determined by the temperature of the field effect transistors used. This is also an undesirable effect.
An alternate approach to the design of a voltage controlled amplifier using field effect transistors in a voltage divider scheme is possible using operational transconductance amplifiers.
An operational transconductance amplifier is basically an electrical device which will produce an output current equal to the product of three terms: a first input current, a second input current, and a constant, K. The first input current is associated with the input signal, and the second input current is associated with the control of the first input current. The constant, K, is a number associated with parameters relating to the characteristics of the particular operational transconductance amplifier used. In particular, the temperature of the operational transconductance amplifier does affect the value of the constant, K.
When a voltage controlled amplifier is implemented using an operational transconductance amplifier, a number of characteristics of interest result.
The dynamic range of the input signal is typically 80 db, with a corresponding range of control of the control signal of approximately 90 db. Furthermore, as an operational transconductance amplifier is an active device, implementing a voltage controlled amplifier using operational transconductance amplifiers produces a controllable device capable of producing gain on the input signal. This is a significant advantage over the previously discussed implementation of a voltage controlled device employing field effect transistors.
However, while the use of operational transconductance amplifiers does so offer advantages over the use of field effect transistors in the implementation of a voltage controllable device, there are a number of disdvantages which result directly from the use of an operational transconductance amplifier.
First, the relation between the control signal and the input signal is a linear one, i.e., the output signal produced by the operational transconductance amplifier is equal to the input signal multiplied by a constant and the value of the control signal. As voltage controlled amplifiers are frequently used in the processing of audio information, an exponential relationship between the input signal and the control signal is desirable.
A second disadvantage present in the use of an operational transconductance amplifier relates to the amount of second harmonic distortion. While distortion of any type is clearly undesirable, second harmonic distortion in an operational transconductance amplifier is related to input offset current and voltage, and can consequently be minimized by adjusting the input offset current to a minimum value. As the amount of second harmonic distortion is related to the magnitude of the input offset current, the resulting amount of second harmonic distortion appearing in the output signal is independent of the amplitude of the input signal. However, this does present the disadvantage that for input signals having a low level, the amount of second harmonic distortion is proportionally larger. This characteristic is one of the major disadvantages to the use of an operational transconductance amplifier in the implementation of voltage controlled amplifier.
A further disadvantage in the use of operational transconductance amplifiers in the implementation of a voltage controlled amplifier relates to the level of input noise. In particular, from a practical standpoint, the lower limit on the useable level for input signals is determined by the input noise of the first operational transconductance amplifier in a voltage controlled amplifier. Consequently this problem is particularly significant with the use of operational transconductance amplifiers with low level signals.
A further disadvantage presented by the use of an operational transconductance amplifier relates to the amount of third harmonic distortion produced. As this results primarily from saturation of the operational transconductance amplifier due to large signal amplitudes, clearly this disadvantage can be limited by restricting the maximum level permitted for the output signal. This effectively limits the maximum amplitude of the output signal produced by the operational transconductance amplifier. This characteristic is likewise considered as one of the major disadvantages of the use of an operational transconductance amplifier in the implementation of a voltage controlled amplifier.
Consequently it is observed that the major disadvantages to the use of an operational transconductance amplifier in the implementation of a voltage controlled amplifier relate to the amount of distortion produced when operating the device close to the upper and lower limits of the input signal. The net effect is a reduction in the dynamic range which can exist on the input signal.
A final disadvantage presented by the use of an operational transconductance amplifier in the implementation of voltage controlled amplifiers is the temperature dependence of the gain of the operational transconductance amplifier. This characteristic is clearly undesirable.